One of the pressing questions facing climate science today is understanding how the Sun’s behavior evolves over time. Since the Sun is the main source of energy for Earth’s atmosphere, comprehending how solar irradiance changes over time is key to interpreting past climatological events. If we went through a period of abnormal temperatures in the past, was it due to changes in the Sun’s radiance, or was it due to some more complex interplay between the components of climate on Earth?

You might think we’re up the creek without a paddle – how are we going to measure solar output centuries and millennia in the past? Fortunately, two facts intervene to save us. First, there exists an empirical link between the solar cycle length (SCL) and its amplitude. The Sun goes through cycles of increased and decreased intensity on an ~11 year timescale. The exact length of this cycle is tied to its amplitude, which means that by measuring the SCLs we can constrain the activity levels of the sun – and hence its total irradiance over time. Second, as it turns out, our ancestors were pretty darned intellectually curious! Among other things, they kept records of the sunspots and aurorae they observed every year; both phenomena are tightly correlated with solar activity. By looking at records of these events, scientists have been able to determine SCLs from many different epochs. Don’t try this at home: solar astronomers and navigators often went blind in the course of their work.

The authors focused their efforts on the Medieval Climate Anomaly (MCA), a period of anomalously high temperatures in medieval Europe ~1000-1300. The traditional explanation for the MCA is increased solar irradiance. However, there are other explanations. For example, volcanic activity is negatively correlated with temperature; as volcanic activity increases, more reflective particles are pumped into the atmosphere, reflecting more sunlight and decreasing temperatures. Another explanation for low temperatures revolves around the internal variability of the ocean-climate system – both the atmosphere and the ocean have natural variability that might have conspired to increase the temperature. The authors aim to test the solar irradiance hypothesis.

Methods and Results

The authors consider two different metrics of solar activity: annual sunspot count (50 events during the MCA), and annual aurora count (246 events during the MCA). Note that these are underestimates of the true counts; our ancestors lacked the telescopes required to get precision measurements. Next, for each metric the authors computed the mean annual number of events and the standard deviation of the sample. They looked for those years in which the number of events exceeded the mean by three standard deviations or more, and termed these the solar cycle maxima. By measuring the interval between the maxima, the authors derived the length of the solar cycle as a function of solar cycle number. They find that the mean SCL is 10.61 ± 0.21 for the auroral record, and 11.50 ± 0.58 for the sunspot. Taken together, the datasets imply a mean SCL of 10.72 ± 0.20. Figure 1 illustrates this fit.

Figure 1: Year of solar cycle maximum vs. solar cycle number. The orange triangles are derived from aurorae, and the blue dots from sunspots. The slope of this line gives us the average SCL (years/solar cycle).

This SCL is well-defined and “not exceptionally anomalous”. It is consistent with the SCL obtained for the more recent period 1870-1979, and disfavors solar forcing as an explanation for the MCA.

Next, the authors move to connect SCL to Total Solar Irradiance (TSI). This is more dicey; it relies on an empirical relationship calibrated in the present, and there is no guarantee that this same relationship was applicable in the past. Assuming it works, you arrive at the plot shown in Figure 2. From this analysis, TSI during the MCA was on the high end of solar activity, but not exceptionally anomalously so. This again disfavors the solar activity hypothesis, suggesting that volcanic activity or the ocean-atmosphere coupling had a stronger causative effect on the MCA.

Figure 2: SCL and TSI versus solar cycle number. TSI is derived from an empirical relationship connecting TSI to SCL. The solid green line is the mean SCL measured from the dataset, with the dashed green lines corresponding to standard error. The black line is the solar irradiance expected from models. The red points are the SCLs determined by looking from maximum to maximum; the blue points represent an alternate methodology that measures from minimum to minimum. This method is less reliable than the maximum-to-maximum method and hence not discussed in this paper (so focus on the red dots!). The broad trend is that while there is variability in the SCLs over the course of the epoch studied, the mean activity level is not exceptionally anomalous.

On a personal note, this paper reads to me like a success story for intellectual curiosity. Our forbearers could not possibly have known the uses to which their sunspot and aurora records would be put. They recorded them just because they thought they were interesting. Now, nearly a millennium hence, we are using their work to help inform our debate on climate change! It looks like it’s true: knowledge is the investment that always pays out, in the long run.

About Sukrit Ranjan

I am a graduate student at Harvard University studying extrasolar planets and astrobiology. In my free time, I like books, board games, and flying over Boston.

http://abstrusegoose.com/308

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3 Comments

Tony George
on March 8, 2012 at 12:12 am

More question than comment: Are you testing for abnormally HIGH or LOW temperatures during the period? If HIGH, then your chart would indicate that TSI is higher during the MCA. If LOW, then TSI is opposite the expected effect during MCA. Your article indicates HIGH.

Just to show once more how complex those things really are – recently the experts from NASA in charge of the calibration of the satellite measurements of TSI have discovered that the older value (from the 1990s) around 1365.4 W/m^2 was inaccurate, and have set a new value around 1360.8 W/m^2 – see the paper from:

Tony: as I understand it, SCL is a proxy for solar activity level, so that longer SCLs are associated with higher solar output and hence temperature. I’m not familiar with the details of climate literature, but the authors indicate the SCLs, while high, are not indicative of an “exceptionally” high TSI that would account for the increased temperatures recorded.